Until the PRESCAP method was used in 1967 to make tree sap extracts, no anti-cancer activity tests had been made with fresh tree sap. From 1967 through 1971 sap was displaced from 544 North American tree species by fitting a PRESCAP on the end of freshly-cut stems. Hot water forced longitudinally through the stems displaced the sap which was evaporated to dryness and the dried extracts tested in the cancer screen. Thirty-one (5.7%) of these extracts showed activity against P388 Leukemia. Most of these active extracts were from species already tested by conventional methods of extraction of leaves, bark, fruits, etc. in which no activity had been found. The difference in activity is because the sap contains the whole spectrum of chemicals ultimately found in individual plant parts and therefore the statistical chances of finding active materials is greater. After exhausting the readily available North American species, the work was moved in 1971 to Costa Rica where there are about 2000 species. Extracts of 770 of these have been made. Screening results have been obtained on the first 200. Thirty-six (18%) show confirmed activity. The proposed grant will be used to obtain extracts from the 1200- odd species remaining with the idea of locating a large back-log of active materials from which the more promising can be selected for large scale production for fractionation and isolation of active principles, looking toward clinical testing in humans.